The overall purpose of this research was to improve the tumor targeting properties of radiolabeled biologicals by optimizing chemical parameters. For the past one year, we have synthesized a second generation radiolabeled peptidomimetic antagonist (IA) with a high affinity for ava3 receptor by conjugating the amino terminus of 4-[2-(3,4,5,6-tetrahydro-pyrimidin-2-ylamino)-ethyloxy]benzoyl-2-(S)-[N-(3-amino-neopenta-1-carbamyl)]-aminoethylsulfonylamino-a-alanine hydrochloride (IAC), a hydrophobic carbamate derivative of the first generation IA, with 2-p-isothiocyanatobenzyl-DOTA and labeled with 111In.. We have demonstrated that this 111In-IAC with a higher affinity to the receptor accumulated in the receptor-positive tumor (xenografted in nude mice) with a higher peak tumor uptake value and was retained in the tumor for a much longer duration than that of the first generation IA with a lower affinity while clearing rapidly from the whole-body via the renal excretion. We have also synthesized oligomeric forms of the first generation IA by conjugating IA to monoclonal antibody (MoAb). The receptor-binding affinity increased proportionally to the level of IA molecules conjugated per MoAb. The fluorescence microscopic analysis showed that fluorescein labeled MoAb-IA10 outlined neovasculatures but not tumor cells at 4 and 21 hr post injection. The neovasculatures outlined with FITC-MoAb-IA10 were superimposed with blood vessels outlined with rhodamine-lectin. This fluorescence microscopic analysis supports the hypothesis that the 111In labeled MoAb-IA10 in tumors was localized on the neovasculatures via binding to ava3 receptors, but not on tumor cells.